Doctrine won't let me select specific fields - symfony

Question

The symfony framework features an app/console file that can be executed via php to perform some maintenance tasks. It allows users to run DQL queries as well:
# php app/console doctrine:query:dql --hydrate=array \
'SELECT u.id, u.nameFirst, u.nameLast FROM DatabaseBundle:User u'
array
0 =>
array
'id' => string '1' (length=1)
'nameFirst' => string 'jaroslav' (length=8)
'nameLast' => string 'rakhmatoullin' (length=13)
1 =>
array
'id' => string '2' (length=1)
'nameFirst' => string 'Båb Kåre' (length=10)
'nameLast' => string 'Ytrefoss' (length=8)
Observe that I selected three specific columns. The problem I'm having is that a similar query gives me an error when two tables are joined.
# php app/console doctrine:query:dql --hydrate=array \
'SELECT u.id , r FROM DatabaseBundle:User u JOIN u.roles r'
[Doctrine\ORM\Query\QueryException]
[Semantical Error] line 0, col -1 near 'SELECT u.id ,':
Error: Cannot select entity through identification variables
without choosing at least one root entity alias.
The following returns the whole user joined with his roles:
# php app/console doctrine:query:dql --hydrate=array \
'SELECT u, r FROM DatabaseBundle:User u JOIN u.roles r'
Obviously, I'm missing something.
Any ideas? I would appreciate links to appropriate docs too (on this specific matter).

Answer 1

From the documentation on "Partial Object Syntax":
By default when you run a DQL query in Doctrine and select only a subset of the fields for a given entity, you do not receive objects back. Instead, you receive only arrays as a flat rectangular result set, similar to how you would if you were just using SQL directly and joining some data.
If you want to select partial objects you can use the partial DQL keyword.
php console doctrine:query:dql --hydrate array \
'SELECT partial s.{name ,id}, partial c.{name, id }
FROM DatabaseBundle:ProductCategories c
JOIN c.suppliers s ORDER BY s.name, c.name'